Ziegler-Natta (ZN) type polyolefin catalysts are well known in the field of polymers, generally they comprise (a) at least a catalyst component formed from a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic Chemistry, 1989), a metal compound of Group 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compound of group 13 of the Periodic Table (IUPAC) and/or an internal donor compound. ZN catalyst may also comprise (b) further catalyst component(s), such as a cocatalyst and/or an external donor.
Various methods for preparing ZN catalysts are known in the state of art. In one known method, a supported ZN catalyst system is prepared by impregnating the catalyst components on a particulate support material. In WO-A-01 55 230, the catalyst component(s) are supported on a porous, inorganic or organic particulate carrier material, such as silica.
In a further well known method the carrier material is based on one of the catalyst components, e.g. on a magnesium compound, such as MgCl2. This type of carrier material can also be formed in various ways. EP-A-713 886 of Japan Olefins describes the formation of MgCl2 adduct with an alcohol which is then emulsified and finally the resultant mixture is quenched to cause the solidification of the droplets.
Alternatively, EP-A-856 013 of BP discloses the formation of a solid Mg-based carrier, wherein the Mg-component containing phase is dispersed to a continuous phase and the dispersed Mg-phase is solidified by adding the two-phase mixture to a liquid hydrocarbon.
The formed solid carrier particles are normally treated with a transition metal compound and optionally with other compounds for forming the active catalyst.
Accordingly, in case of external carriers, some examples of which are disclosed above, the morphology of the carrier is one of the defining factors for the morphology of the final catalyst.
One disadvantage encountered with the supported catalyst systems is that distribution of the catalytically active compounds on the support material is dependent on the surface chemistry and the surface structure of the support material. As a result this may often lead to non-uniform distribution of the active component(s) within the catalyst particle. As a consequence of the uneven distribution of the active sites in catalyst particles catalysts with intra-particle in-homogeneities, as well inter-particle in-homogeneities between separate particles are obtained, which leads finally to in-homogeneous polymer material.
Further, support material will remain in the final polymer as a residue, which might be harmful in some polymer applications.
WO-A-00 08073 and WO-A-00 08074 describe further methods for producing a solid ZN catalyst, wherein a solution of an Mg-based compound and one or more further catalyst compounds are formed and the reaction product thereof is precipitated out of the solution by heating the system. Furthermore, EP-A-926 165 discloses another precipitating method, wherein a mixture of MgCl2 and Mg-alkoxide is precipitated together with a Ti-compound to give a ZN catalyst.
According to US 2005/0176900 a magnesium compound, an alcohol, an ether, a surfactant and an alkyl silicate are reacted first to get a catalyst support, which is then further reacted with a titanium compound. The solid titanium catalyst component is obtained via precipitation. The catalyst component further comprises an internal donor, which can be selected form a great variety of compounds.
WO 03/000757 as well WO 03/000754 describe a process for the preparation of an olefin polymerization catalyst component, enabling to prepare solid particles of a catalyst component comprising a group 2 metal together with a transition metal however without using any external carrier material or without using conventional precipitation methods, but using so called emulsification-solidification method for producing solid catalyst particles. In this process a phthalate type internal electron donor is prepared in situ during the catalyst preparation in a way and using chemicals so that an emulsion is formed. Droplets of the dispersed phase of the emulsion form the catalyst component, and solidifying the droplets results in solid particulate catalyst.
WO 2004/029112 discloses a further modification of the emulsion-solidification method as described in WO 03/000757 as well WO 03/000754, and relates thus to process for preparing an olefin polymerization catalyst component, wherein the process is further characterized in that a specific aluminum alkyl compound is brought into contact with the catalyst component, enabling a certain degree of activity increase at higher temperatures.
The above described ZN-catalysts are described to be useful in olefin polymerization, i.e. for producing propylene polymers.
In view of producing polyethylenes one problem that is often encountered with prior art ZN-catalysts is that it is difficult to produce an ethylene homo- or copolymer having high molecular weight. Polyethylenes of high molecular weight show i.a. an enhanced toughness. However, as the molecular weight of the polyethylene increases, the processability of the resin usually decreases.
Another problem encountered with prior art catalysts is that their activity is not very high.
Accordingly, although much development work has been done in the field of Ziegler-Natta catalysts for producing polymers, there remains a need for alternative or improved methods of producing ZN procatalysts with desirable properties for producing ethylene polymers with desired properties, since the properties of the ethylene (co-)polymers are i.a. determined by the catalyst used for their production.
Thus, it was an objective of the present invention to provide a process for preparing a procatalyst of a Ziegler-Natta catalyst composition which allows for the production of polyethylene having improved and more controlled properties. In particular it is desirable that the catalyst allows for a “tailoring” of the properties of the produced resins.
For tailoring the properties of the produced ethylene polymers it was already suggested to use ZN-catalysts comprising an internal donor, like THF (tetrahydrofuran).
For example WO 00/58374 discloses ZN catalysts comprising THF, whereby the primary purpose of the THF is to reduce the level of electrostatic charge in the polymerization medium so that agglomeration of polymer particles does not occur.
U.S. Pat. No. 5,055,535 discloses a method for controlling the MWD of polyethylene homopolymers and copolymers using a ZN catalyst comprising an electron donor selected from monoethers (e.g. tetrahydrofuran). The monoether, like tetrahydrofuran is added to the catalytic component and the cocatalyst, at the latest, upon commencement of the polymerization reaction and that under no circumstance should the monoethers be brought into contact with the catalytic component without the presence of the cocatalyst in the medium.
EP 1780225 A1 suggest the possibility of tailoring the properties of a multimodal ethylene polymer by using a modified ZN catalyst to influence the molecular weight distribution (MWD) of a higher molecular weight (HMW) component whilst essentially having no affect on the MWD of the lower molecular weight (LMW) component. The electron donor present in the ZN catalysts used is preferably tetrahydrofuran, whereby the ZN catalyst is preferably pre-formed and then contacted with an electron donor and optionally dried.
Nowadays HSE-(health, safety & environment) policies are an important factor in the production of catalysts and further polymers. In other words the polymers must fulfill the strict health and environmental requirements national and international institutions. One class of substances which have been considered as potential harmful compounds is phthalates, which have been commonly used as internal electron donors in Ziegler-Natta type catalysts. Also tetrahydrofuran has been recognized as a hazardous substance.
For these reasons it is still desirable to find alternative internal donors which do not include phthalates and/or tetrahydrofuran and which yields desired polymer properties, namely high molecular weight.
It would be furthermore highly advantageous if a process for preparing solid olefin polymerization catalyst components, i.e. the procatalyst, would be available which allows the formation of said solid catalyst components in different ways, like via precipitation or emulsion/solidification method, depending on the desired properties of the catalyst particles, i.e. desired morphology and/or particle size, whereby no gel-like material is formed during catalyst preparation and whereby the produced catalyst results in desired polymer properties, like melt flow rate, Mw, melting point, etc. The different ways (e.g. precipitation or emulsion/solidification method) thereby show a common mechanism.
Surprisingly these objects could be solved by the use of special bi-(oxygen containing ring) compounds as internal donor, which is added during the catalyst preparation.
Such bi-(oxygen containing ring) compounds have the formula (I)
    wherein in the formula (I)    X is C or Si,    R1 can be the same or different and can be hydrogen, a linear or branched C1 to C8-alkyl group, or a C3-C8-alkylene group,    and R2 to R5 are the same or different and can be hydrogen, a linear or branched C1 to C8-alkyl, or a C3-C8-alkylene group,    whereby two or more of R1 to R5 can form a ring,    n is the same for both oxygen-containing rings and can be 0, 1 or 2,    whereby the two oxygen-containing rings can be individually saturated or unsaturated.
These compounds are known for example from EP 2495266 A1 or WO-2011157742 as randomizer/polar agent in the preparation of high styrene high vinyl solution-based styrene-butadiene rubber or from EP 183538 A1 as a reactive diluent in epoxy resins.